A capacitor is a passive electronic component that is used to store energy in the form of an electrostatic field, and comprises a pair of electrodes separated by a dielectric layer. When a potential difference exists between two electrodes, an electric field is present in the dielectric layer. This field stores energy, and an ideal capacitor is characterized by a single constant value of capacitance which is a ratio of the electric charge on each electrode to the potential difference between them. In practice, the dielectric layer between electrodes passes a small amount of leakage current. Electrodes and leads introduce an equivalent series resistance, and dielectric layer has limitation to an electric field strength which results in a breakdown voltage. The simplest energy storage device consists of two parallel electrodes separated by a dielectric layer of permittivity ε, each of the electrodes has an area S and is placed on a distance d from each other. Electrodes are considered to extend uniformly over an area S, and a surface charge density can be expressed by the equation: ±ρ=±Q/S. As the width of the electrodes is much greater than the separation (distance) d, an electrical field near the center of the capacitor will be uniform with the magnitude E=ρ/ε. Voltage is defined as a line integral of the electric field between electrodes. An ideal capacitor is characterized by a constant capacitance C defined by the formula:C=Q/V,  (1)which shows that capacitance increases with area and decreases with distance. Therefore the capacitance is largest in devices made of materials of high permittivity.
A characteristic electric field known as the breakdown strength Ebd, is an electric field in which the dielectric layer in a capacitor becomes conductive. Voltage at which this occurs is called the breakdown voltage of the device, and is given by the product of dielectric strength and separation between the electrodes:Vbd=Ebdd  (2)
The maximal volumetric energy density stored in the capacitor is limited by the value proportional to ˜ε·E2bd, where ε is dielectric permittivity and Ebd is breakdown strength. Thus, in order to increase the stored energy of the capacitor it is necessary to increase dielectric permeability c and breakdown strength Ebd of the dielectric.
For high voltage applications much larger capacitors have to be used. There are a number of factors that can dramatically reduce the breakdown voltage. Geometry of the conductive electrodes is important for these applications. In particular, sharp edges or points hugely increase the electric field strength locally and can lead to a local breakdown. Once a local breakdown starts at any point, the breakdown will quickly “trace” through the dielectric layer till it reaches the opposite electrode and causes a short circuit.
Breakdown of the dielectric layer usually occurs as follows. The intensity of the electric field becomes high enough to free electrons from atoms of the dielectric material and the dielectric material begins to conduct an electric current from one electrode to another. Presence of impurities in the dielectric or imperfections of the crystal structure can result in an avalanche breakdown as observed in semiconductor devices, such as avalanche diodes and avalanche transistors.
Another important characteristic of a dielectric material is its dielectric permittivity. Different types of dielectric materials are used for capacitors and include ceramics, polymer film, paper, and electrolytic capacitors of different kinds. The most widely used polymer film materials are polypropylene and polyester. An important aspect of dielectric permittivity is that an increase of dielectric permittivity increases the maximum volumetric energy density that can be stored in a capacitor.
A material with an ultra-high dielectric constant was found to be the composite polyaniline, PANI-DBSA/PAA. PANI-DBSA/PAA is synthesized using in situ polymerization of aniline in an aqueous dispersion of poly-acrylic acid (PAA) in the presence of dodecylbenzene sulfonate (DBSA) (Chao-Hsien Hoa et al., “High dielectric constant polyaniline/poly(acrylic acid) composites prepared by in situ polymerization”, Synthetic Metals 158 (2008), pp. 630-637). The water-soluble PAA serves as a polymeric stabilizer, protecting the PANI particles from macroscopic aggregation. A very high dielectric constant of ca. 2.0*105 (at 1 kHz) is obtained for the composite containing 30% PANI by weight. SEM micrograph reveals that composites with high PANI content (i.e., 20 wt. %) consisted of numerous nano-scale PANI particles that are evenly distributed within the PAA matrix. The individual nano-scale PANI particles may be thought of as small capacitors. (Hoa et al.) attribute high dielectric constants to the sum of the small capacitors corresponding to the PANI particles. A major drawback of this material is a possible occurrence of percolation and formation of at least one continuous conductive path under electric field with probability of such an event increasing with an increase of the electric field. When at least one continuous path (track) through the neighboring conducting PANI particles is formed between electrodes of the capacitor, it decreases a breakdown voltage of the capacitor.
Single crystals of doped aniline oligomers are produced via a simple solution-based self-assembly method (see, Yue Wang, et. al., “Morphological and Dimensional Control via Hierarchical Assembly of Doped Oligoaniline Single Crystals”, J. Am. Chem. Soc. 2012, 134, pp. 9251-9262). Detailed mechanistic studies have revealed that crystals of different morphologies and dimensions can be produced by a “bottom-up” hierarchical assembly where structures such as one-dimensional (1-D) nanofibers can be aggregated into higher order architectures. A large variety of crystalline nanostructures, including 1-D nanofibers and nanowires, 2-D nanoribbons and nanosheets, 3-D nanoplates, stacked sheets, nanoflowers, porous networks, hollow spheres, and twisted coils, can be obtained by controlling the nucleation of the crystals and the non-covalent interactions between the doped oligomers. These nanoscale crystals exhibit enhanced conductivity compared to their bulk counterparts as well as interesting structure-property relationships such as shape-dependent crystallinity. Furthermore, the morphology and dimension of these structures can be largely rationalized and predicted by monitoring molecule-solvent interactions via absorption studies.
There is a known energy storage device based on a multilayer structure. The energy storage device includes first and second electrodes, and a multilayer structure comprising blocking and dielectric layers. The first blocking layer is disposed between the first electrode and a dielectric layer, and the second blocking layer is disposed between the second electrode and a dielectric layer. Dielectric constants of the first and second blocking layers are both independently greater than the dielectric constant of the dielectric layer. FIG. 1 shows one exemplary design that includes electrodes 1 and 2, and multilayer structure comprising layers made of dielectric material (3, 4, 5) which are separated by layers of blocking material (6, 7, 8, 9). The blocking layers 6 and 9 are disposed in the neighborhood of the electrodes 1 and 2 accordingly and characterized by higher dielectric constant than dielectric constant of the dielectric material. A drawback of this device is that blocking layers of high dielectric permittivity located directly in contact with electrodes can lead to destruction of the energy storage device. Materials with high dielectric permittivity which are based on composite materials and contain polarized particles (such as PANI particles) might demonstrate a percolation phenomenon. The formed polycrystalline structure of layers has multiple tangling chemical bonds on borders between crystallites. When a material with a high dielectric permittivity possesses a polycrystalline structure is used a percolation might occur along the borders of crystal grains. Another drawback of current devices is that they require the expensive manufacturing procedure of vacuum deposition of all layers.
Capacitors as energy storage devices have well-known advantages versus electrochemical energy storage, e.g. a battery. Compared to batteries, capacitors can store energy with very high power density, i.e. charge/recharge rates, have long shelf life with little degradation, and can be charged and discharged (cycled) hundreds of thousands or millions of times. However, capacitors often do not store energy in a small volume or weight as in a case of batteries, or at low energy storage cost, which makes capacitors impractical for some applications, for example electric vehicles. Accordingly, it would be an advance in energy storage technology to provide capacitors of higher volumetric and mass energy storage density and lower cost.
The present invention solves a problem of the further increase of volumetric and mass density of reserved energy of the capacitor, and at the same time reduces cost of materials and manufacturing process.